While a variety of data storage mediums are available, magnetic tape remains a preferred technology for economically storing large amounts of data. To facilitate the efficient use of this particular magnetic medium, magnetic tape is widely used in so-called multi-channel, linear tape format in which a plurality of servo and data tracks extend in the longitudinal direction of the tape. Magnetic track transitions embodying recorded data or servo information may be written into tracks with a variety of orientations.
After one or more write elements record data on the tape, one or more data heads containing one or more read elements will read the data from those tracks as the tape advances in the longitudinal or transducing direction in which the magnetic transitions move past the head to be read back. It is generally not feasible to provide a dedicated head or element for each data track, therefore, a multi-channel head(s) must move across the width of the tape, and each data channel head element accesses a large number of data tracks dedicated specifically to that head channel element. At each track the element must be accurately centered over the data track. In very high density cases this cannot be achieved by mechanical means alone, and a track following servo is employed where dedicated servo read elements read a band of prerecorded servo tracks which correspond to specific data tracks in each data band associated with a given data (read or write) element. The servo band is used in controlling the translational movement of the head(s). The servo band is used not only to position the head(s) on the correct track but also to keep it on the track once it has arrived at the track.
The servo track contains data, which when read by the servo read element, is indicative of the relative position of the servo read element with respect to the magnetic media in a translating direction. In one type of traditional amplitude based servo arrangement, the servo track is divided in half. Servo data is recorded in each half track, at different frequencies. The servo read element is approximately as wide as the width of a single half track. Therefore, the servo read element determines its relative position by moving in a translating direction across the two half tracks. The relative strength of a particular frequency of servo signal would indicate how much of the servo read element is located within that particular half track. The trend toward thinner and thinner magnetic tape layers causes amplitude modulation problems with this and other amplitude based heads. That is, as the thickness of the magnetic layer decreases, normal variations on the surface represent a much larger percentage variation in the magnetic layer, which may dramatically affect the output signal.
One type of servo control system was created which allows for a more reliable positional determination by reducing the amplitude based servo signal error traditionally generated by debris accumulation, media thickness non-uniformity and head wear. U.S. Pat. No. 5,689,384 (Albrecht, Barrett, and Eaton, IBM), incorporated herein by reference in its entirety, describes using a timing-based servo pattern on a magnetic recording head.
In a timing-based servo pattern, magnetic marks (transitions) are recorded in pairs within the servo track. Each mark of the pair is angularly offset from the other. For example, a diamond pattern has been suggested and employed with great success. The diamond extends across the servo track in the translating direction. As the tape advances, the servo read element detects a signal or pulse generated by the first edge of the first mark. Then, as the element passes over the second edge of the first mark, a signal of opposite polarity will be generated. Now, as the tape progresses, no signal is generated until the first edge of the second mark is reached.
Once again, as the element passes the second edge of the second mark, a pulse of opposite polarity is generated. This pattern is repeated indefinitely along the length of the servo track.
Therefore, after the element has passed the second edge of the second mark, it arrives at another pair of marks. The time it took to move from the first mark to the second mark is noted. Additionally, the time it takes to move from the first mark (of the first pair) to the first mark of the second pair is similarly noted.
The ratio of these two time components is indicative of the position of the read element within the servo track, in the translating direction. As the read head moves in the translating direction, this ratio varies continuously because of the angular offset of the marks. It should be noted that the servo read element is relatively small compared to the width of the servo track. Ideally, the servo element is smaller than one half the width of a written data track. Because position is determined by analyzing a ratio of two time/distance measurements, taken relatively close together, the system is able to provide accurate positional data, independent of the absolute speed of the media. In such systems, the variations in the speed need to be relatively well controlled.
Once the position of the servo read element is accurately determined, the position of the various data read elements can be controlled and adjusted with a similar degree of accuracy on the same substrate. Namely, the various read elements are fabricated on the same substrate with a known and, generally, the same spacing between them. Hence, knowing the location of the servo element allows for a determination of the location of all the data elements.
When producing magnetic tape, or any other magnetic media, the servo track is generally written by the manufacturer. This results in a more consistent and continuous servo track, over time. To write the timing-based servo track described above, a magnetic recording head bearing the particular angular pattern as its gap structure is utilized. To achieve maximum accuracy in the servo positioning signal, it is necessary to write a very accurate servo pattern. This means that a very precise servo recording element must be fabricated.
In the case of azimuthal recording schemes for linear multi-channel tape, as disclosed in the Large Angle Azimuthal Recording System (“LAAZR”) patents applied for by Schwarz and Dugas, having Ser. No. 10/793,502, filed Mar. 4, 2004, which are incorporated in their entirety by reference, there exists a need for arbitrary shaped gaps for the servo writing elements, as well as the write and read elements, to have large angle gap features. This later can be addressed by making a large angle mechanical placement of non-angular thin film head row bars into a slider assembly. The proposed head of this invention may simplify the need for the large angle mechanical placement and result in a simpler slider assembly, in particular for the write head of such a system.
Two general types of recording heads, each having the capability of multiple arbitrary slanted gap features, such as those for timing-base servo patterns on tape media, are generally known. One type is a ferrite composite substrate assembly with a horizontal surface film process and the other type is that of a horizontally processed pure integrated thin film head.
The first type, perhaps the most simple, is a ferrite ceramic composite structure as disclosed in U.S. Pat. No. 5,689,384 (Albrecht, Barrett, and Eaton, IBM), in U.S. Pat. No. 6,269,533 (Dugas, ARC) and U.S. Pat. No. 6,496,328 (Dugas, ARC).
The second type, a pure horizontal planar process thin film head, is disclosed by Aboaf, Dennison, Friedman, Kahwaty, and Kluge in U.S. Pat. No. 5,572,392 and in U.S. Pat. No. 5,652,015. In these patents, the process is referred to as a single major plane process. That process is referred to herein as Horizontal Planar Process (“HPP”) since the plane of processing in that head substrate lies parallel to the tape bearing surface. Indeed the first type, the Albrecht reference and the Dugas reference heads also use the HPP approach; however, those heads are not fully integrated and use a composite ferrite/ceramic substrate structure with a wound coil.
With a pure integrated thin film head, all of the components of the head are created from depositing and patterning different layers of materials, as thin films, generally on a substrate. For example, the magnetic core, the windings and any low permeability barrier materials are formed by producing thin films. In some designs which employ a magnetic substrate or wafer, such as Ni—Zn ferrite, this magnetic substrate may end up as a shield or a pole or as part of a magnetic yoke.
The integrated thin film head design and process of Aboaf is capable of multiple arbitrary slanted gaps as required of timing-base servo systems precisely because of the horizontal planar process used in that head construction. While this head solved the arbitrary gap limitation of the standard thin film head industry process, such a head is extremely difficult to manufacture and has not been produced commercially.
The typical integrated thin film tape or disk head process is herein referred to as a Vertical Planar Process (“VPP”) since the plane of processing in that wafer is perpendicular or vertical to the tape bearing surface. This process is used almost exclusively in the thin film head industry. The VPP technique as used in data heads, as is easily understood from the referenced patents, cannot make slanted gaps or pairs of oppositely slanted gaps as required by timing-base servo heads and complex azimuthal recording schemes. Hence, to date, pure thin film heads such as those that are made from VPP techniques are not suitable for timing-based heads, and those made from a fully integrated HPP technology are not seen as practical to produce such a magnetic head, each for different reasons.
FIG. 1A is a side cross sectional view that shows a prior art conventional thin film VPP magnetic data head 100 for use in data recording on magnetic media such as disks or tape. This head consists of a generally non-magnetic substrate 110, a layer of polished alumina 111, a sputtered or plated first magnetic pole 112, an insulating gap layer 120 which is generally alumina, coils 118, an insulating layer 117 which encompasses coils 118, a second magnetic pole piece 114, a planarized overcoat layer 122, and typically in the case of tape heads, a nonmagnetic closure piece 124. A magnetic tape medium 126 moves in a direction as shown by arrow 128 operating in a motion transverse to the poles pieces 112 and 114 and over a bearing surface 129. The head is lapped to a gap depth 148 which is the distance from the tape bearing surface 129 to the apex point 121 of the second pole 114 usually involving the use of lapping guides, made during the wafer fabrication process. Direction arrow 185 shows the direction of film layer growth from the wafer substrate surface.
Typically, VPP heads cannot be manufactured with a set of angled gaps as required for timing-based servo heads. This is shown in FIG. 1B which is a top view of the prior art of FIG. 1A. The gap 120 of such a process is essentially planar or parallel to the wafer 110 surface. This makes it difficult, if not impossible, to make an angled gap, and the extension to multiple angled gaps in one head channel seems even more improbable.
An integrated horizontal magnetic head design solves this limitation of planar gaps. This head as shown in the cross section of FIG. 2A and the top view of FIG. 2B, takes advantage of processing in a different major plane from the head of FIG. 1. This head uses an HPP wafer construction to distinguish it from the VPP wafer construction of the thin film head of FIG. 1.
With such an HPP approach, the arbitrary gap structures required for timing-base servo systems can be realized. In addition to the previous cited Aboaf patents '392 and '015, other examples of this type of head include the head of U.S. Pat. No. 4,837,924, Jean-Pierre Lazzari, issued on Jun. 13, 1989, and titled “Process For The Production Of Planar Structure Thin Film Magnetic Recording Head,” and the head of U.S. Pat. No. 5,768,070, by Krounbi and Re, issued on Jun. 16, 1998, and titled “Horizontal Thin Film Write, MR Read Head.” These types of heads are sometimes referred to as “horizontal heads” in the industry.
Head 200 is illustrated in FIG. 2A, the magnetic back yoke 236 is either a ceramic magnetic substrate or a deposited magnetically permeable layer on a ceramic substrate. One write element 232 is shown in cross section. Coil 238 is shown in a 4 turn configuration. Horizontal top poles 242a and 242b conduct the flux from the back yoke 236 to the main recording gap 234 at the surface of the head. Feature 240 is a coil insulating layer. Typically a hard nitrided layer is used for the upper magnetic film 242b. Gap 234 can be defined on any arbitrary angle as shown in the top planar view of FIG. 2B. Direction arrow 285 shows the direction of film growth from the substrate surface in FIG. 2A and FIG. 2B. The recording medium is shown as 226 and moves in the direction as shown by the arrow 228 in both FIGS. 2A and 2B.
Regarding the top view of FIG. 2B, the arbitrary gaps as shown are angled 234b and straight 234a. Coil 238 is shown in a 4 turn configuration. Each of the write elements 232 are shown coupled to a common coil 238. The tape span 226 is shown to illustrate that head support structure and coil 238 extend outside the tape path and allows for the leads to be attached to bond pads 237.
The head just described in FIGS. 2A and 2B has the flexibility of arbitrary gap angles. However, it is not clear that the standard ion milling technique proposed to etch the gaps will result in good gap wall definition due to the well known aspect ratio considerations in such a milling technique. Moreover, from a consideration of the layout of the coil upon the surface plane as shown in FIG. 2B, it may be difficult to have each write element 232 independently addressable with a separate coil. Bringing coil terminations and leads out of the tape bearing plane of the head may pose a design issue for the head assembly.
The surface film heads used commercially today for servo tape formatting of arbitrary angles gaps are made of structures and techniques proposed by the heads shown in FIGS. 3 and 4, respectively. These heads are made with a horizontal planar or surface thin film process in combination with a ferrite/ceramic composite substrate. The substrate carries the subgap embedded within it. The heads, as taught in '384, and in '328, are practical heads used to make arbitrary slanted gaps. These types of heads are referred to herein as composite ferrite/ceramic surface film heads.
Head 300 is illustrated in the prior art FIG. 3A. The composite ferrite/ceramic surface film head 300 includes two ferrite blocks 308, 306 that are bonded to a ceramic member 311 that extends the entire width of the head 300. Surface 390 is contoured and polished in preparation for film deposition. A magnetically permeable thin film 304 is deposited over an upper surface 390 of the ferrite blocks 308, 306 and the exposed upper portion of the ceramic member 311. Air slots 312 serve to reduce air entrainment of the tape.
Servo writing gap patterns 314 are formed in the thin film 304, in a well defined arbitrary gap pattern. Winding 320 is wound around 306 and is electrically driven to produce magnetic flux around the ferrite core 306 and through the thin film 304. The flux leaks from the gaps 314 and writes media (not shown) passing over it.
The detail of the gap structure is shown in FIG. 3B. Gap pattern 314 is composed of angled gaps 330 and pattern termination feature 332.
This head has a rather large inductance and, therefore, relatively slow write current rise time. It is also a single drive element design that serves to drive two or more servo elements made into the magnetic film 304 spanning over the subgap formed by ceramic member 311. In this head flux can leak around the gap pattern as the flux is not well confined to the recording gaps 314 unless the head is driven to saturation with extremely high current levels.
The inductance related rise time issues, the lack of independently driven write elements, and the writing uniformity issues were addressed successfully by the prior art head design of patent '328. This head 400 is illustrated in FIG. 4A and in detail in FIGS. 4A-4D.
Head 400 of FIGS. 4A-4D is made of a complex ferrite ceramic composite structure as shown in FIG. 4A. As shown in the detail of single element 450 of FIG. 4B, ferrite core pieces 408 and 406 form the driving poles about a ceramic I-bar which serves as a non-magnetic subgap, 411. Air bleed slots 412 are shown. The subgap 411 in combination with the ferrite subpoles and ferrite back bar closure 430 form an efficient magnetic circuit, which when energized with an electrical current in coil 420, drives magnetic flux through the highly permeable surface thin film layer 404, which spans the subgap from one ferrite member to the other. The flux that is driven across this surface film intercepts the arbitrary shaped gaps 414 that have a stray field that impresses flux onto the recording medium. The detailed shape of the gaps 414, as seen in FIG. 4C, and the write current waveform determine the marks that are recorded onto the medium. As seen in FIG. 4A, ceramic members 440 serve as non-magnetic element spacers and in one embodiment can serve as a tape bearing surface. Glass bond area 441 of FIG. 4C and FIG. 4D, separates the active head element 450, of FIG. 4B and FIG. 4C, from the ceramic spacers 440 on either side as shown in FIG. 4C. The gap pattern 414 may be formed as part of the plating process of the surface film 404, or they may be formed in a subsequent photolithographic etch or other etch processes such as that taught in the '533 patent. The surface film may be etched or deposited into two non-interacting parts. Surface film 404 is magnetically active as it is part of the active head element 450. As shown in FIG. 4D, surface film 405, although of the same deposit as surface film 404, is inactive as it has been separated from 404 by slot 407. Slot 407 may be created by broad beam ion milling or by selective plating. Isolation element slot 407 is typically 30 to 70 microns in width and serves to completely decouple 405 from 404 and to render 405 inactive as a magnetic flux conduction element. However, film 405 does act as a tape bearing member. Such details may be seen in the cross-section of FIG. 4D. This is the subject of the Wear Pads patent application which is a continuation of '528.
Therefore, with full consideration of the background art described, it is desired to find a way to make an even more efficient multi-element servo head that will have even lower inductance and, hence, higher frequency capability and which will serve as a superior platform for the manufacture of complicated multi-gap structures envisioned in the future of magnetic servo tracks for high track density tape products.